1. Field of the Invention
The present invention relates to a tunable-wavelength surface emitting laser, and an optical coherence tomography measuring apparatus using the surface emitting laser.
2. Description of the Related Art
Much research has been performed as of recent regarding tunable-wavelength lasers, of which the wavelength of the emitting light is tunable, since applicability to various fields such as communication, sensing, imaging, and so forth, is anticipated.
One known type of tunable-wavelength laser is a tunable-wavelength surface emitting laser element where the laser oscillation wavelength of a vertical cavity surface emitting laser (hereinafter may be abbreviated to “VCSEL”) is controlled using a mirror having a cantilever structure, disclosed in Japanese Patent Laid-Open No. 9-270556.
A VCSEL is generally configured such that an active layer is disposed between a pair of distributed Bragg reflectors (hereinafter may be abbreviated to “DBR”). Laser oscillation occurs at a wavelength corresponding to the cavity length, which is determined by the distance between the pair of DBRs.
The laser element disclosed in Japanese Patent Laid-Open No. 9-270556 is operable to change the laser oscillation wavelength by changing the cavity length, which is performed by mechanically moving the position of the mirror having the cantilever structure. Hereinafter, a VCSEL of which the wavelength of emitting light can be changed may also be referred to as a “tunable-wavelength VCSEL”.
U.S. Patent Laid-Open No. 2007/0183643 discloses that a tunable-wavelength VCSEL is suitable as a light source for optical coherence tomography (hereinafter may be abbreviated to “OCT”).
A broad range of tunable wavelengths is preferable in a case of using a tunable-wavelength VCSEL as the light source for OCT, to improve depth resolution for OCT. Further, fast wavelength tunability is desirable to reduced OCT measurement time.
However, the present inventor has found problems to be solved regarding tunable-wavelength VCSEL.
In order to broaden the tunable wavelength range for tunable-wavelength VCSEL, the amount of displacement where an upper reflecting mirror or lower reflecting mirror is mechanically moved needs to be increased. If the spring constant of the reflective mirror is too great at this time, great force has to be applied to obtain a great displacement amount, so the spring constant preferably is not great.
Also, in order to speed up the wavelength tunability for tunable-wavelength VCSEL, the reflecting mirror has to be vibrated fast. Raising the resonance frequency for the reflecting mirror to be vibrated is effective.
That is to say, in order to realize both a broader range of tunable wavelength and faster wavelength tunability for tunable-wavelength VCSEL, the resonance frequency needs to be raised without raising the resonance frequency of the reflecting mirror.
The relation between spring constant and resonance frequency is as follows. The spring constant k and resonance frequency f of a reed-shaped cantilever can each be expressed as set forth in Expressions (1) and (2),
                    k        =                                            w              *                              d                3                                                    4              *                              1                3                                              ⁢          E                                    (        1        )                                f        =                  0.56          ⁢                      d                          l              2                                ⁢                                    E                              12                ⁢                                                                  ⁢                ρ                                                                        (        2        )            where w represents the width of the cantilever, d represents the thickness thereof and l the length thereof, E the Young's modulus of the material of which the cantilever is formed, and ρ the density of this material.
It can be understood from the Expressions (1) and (2) above, that the resonance frequency can be increased without having a great spring constant by forming a thin and short cantilever. More specifically, the resonance frequency can be double without changing the spring constant by halving the length and thickness of the cantilever.
However, it is difficult to actually realize such a configuration with a common tunable-wavelength VCSEL.
FIG. 13 illustrates a schematic cross-sectional view of a common tunable-wavelength VCSEL. An active layer 1320 and a gap 1330 are provided between an upper reflecting mirror 1300 and a lower reflecting mirror 1310. Reference numerals 1360 and 1370 denote a first spacer layer and a second spacer layer, respectively.
Moving the upper reflecting mirror 1300 vertically as to the plane of the drawing in FIG. 13 changes the length of the gap 1330 (i.e., the distance between the upper reflecting mirror 1300 and the second spacer layer 1370), thereby changing the cavity length, so the laser oscillation wavelength can be changed.
A DBR having a multilayer configuration of a dielectric or semiconductor is commonly used as the reflecting mirrors, to obtain high reflectance necessary for laser oscillation.
A common DBR is formed by alternately laying two types of layers having different refractive indices, at ¼ wavelength in optical thickness. The refractive index of the DBR is decided by the refractive index difference of the two types of layers and the number of layers. The greater the necessary reflectance is, the greater the number of layers required.
In particular, in a case of forming a DBR using a semiconductor, which does not afford as great a refractive index difference as a dielectric, a great number of layers are needed to obtain a high reflectance required of a reflecting mirror in a VCSEL. Depending on the refractive index of the material, the thickness may be several microns or greater.
As described above, reducing the thickness of the DBRs is desirable from the perspective of raising the resonance frequency without raising the spring constant. Realizing a thinner DBR requires forming each layer making up the DBR thinner, or reducing the number of layers.
The thickness of each layer making up the DBR is designed so that the optical thickness is ¼ the optical wavelength. That is to say, the thickness of each layer is decided by the wavelength of the light to be reflected, and the refractive indices of the materials used, so these cannot be changed very much.
Reducing the number of layers of the DBR reduces the reflectance and leads to an increased laser oscillation threshold, so the number of layers of the DBR cannot be reduced very much.
Accordingly, in practice it is difficult to reduce the thickness of the DBR, and there is a limit to how far the resonance frequency can be increased without increasing the spring constant. Thus, it has been difficult with variable-length VCSELs according to the related art to realize both a broader range of tunable wavelength and faster wavelength tunability.